1. Field of the Invention
The present invention relates to an image reading apparatus, and particularly to a technique that corrects for a variation in light quantity from a light source in an image reading apparatus.
2. Description of the Related Art
In image reading apparatus in general, shading correction is performed in order to correct for differences between outputs from an image sensing element at different main scanning positions or for differences in the quantity of light from a light source at different main scanning positions. Shading correction is performed by reading a white reference plate whose density is controlled and managed so as to be uniform.
Meanwhile, in recent years, there is a demand for continuous reading of a large number of originals. It may take several minutes to several tens of minutes to read a large number of originals. The longer the reading time, the longer the time for which the light source is driven. If the driving time of the light source increases, the quantity of light in a middle phase or a later phase of reading may vary relative to the quantity of light in an early phase of reading. The temporal variation of light quantity may occur not only with rare gas fluorescent lamps but also with LED arrays. It should be noted that the quantity of light may decrease at a constant rate irrespective of the main scanning position or may decrease at different rates depending on the main scanning position. In particular, the former situation may occur when only a temporal variation of light quantity occurs, and the latter situation may occur when a temporal variation of light quantity and a spatial variation of light quantity occur. The spatial variation of light quantity sometimes is called a variation of light distribution.
Generally, the white reference plate is provided in a position different from a position at which an image is read (image reading position). Therefore, the white reference plate cannot be read during continuous reading. To address this issue, Japanese Patent Laid-Open No. 2006-148259 proposes correcting for a variation of light quantity based on the results of reading the surface of a platen roller provided in the image reading position at constant intervals, instead of the white reference plate.
The invention disclosed in Japanese Patent Laid-Open No. 2006-148259 is very advantageous because the variation of light quantity can be corrected for without interrupting continuous reading. However, there is a risk of miscorrection if the surface of the platen roller or a platen glass is contaminated with ink or the like. This is because of the assumption that the surface of the platen roller is always maintained at the same density. Accordingly, the invention disclosed in Japanese Patent Laid-Open No. 2006-148259 may not be able to sufficiently deal with such unexpectedly occurring contamination of the surface of the platen roller.